Inkjet printing is a non-impact method for producing printed images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital signals. There are various methods that may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired printed image. In one process, known as drop-on-demand inkjet, individual droplets are projected as needed onto the image-recording element to form the desired printed image. Common methods of controlling the ejection of ink droplets in drop-on-demand printing include thermal bubble formation (thermal inkjet (TIJ)) and piezoelectric transducers. In another process known as continuous inkjet (CIJ), a continuous stream of droplets is generated and expelled in an image-wise manner onto the surface of the image-recording element, while non-imaged droplets are deflected, caught and recycled to an ink sump. Inkjet printers have found broad applications across markets ranging from desktop document and photographic-quality imaging, to short run printing and industrial labeling.
Ink compositions containing colorants used in inkjet printers can be classified as either pigment-based, in which the colorant exists as pigment particles suspended in the ink composition, or as dye-based, in which the colorant exists as a fully solvated dye species that consists of one or more dye molecules. Pigments are highly desirable since they are far more resistant to fading than dyes. However, pigment inks can have inferior durability after printing, especially under conditions where abrasive forces have been applied to the printed image and especially at short time intervals from immediately after printing to several minutes while the inks are drying.
Pigment inks can be difficult to jet through ink jet print heads having small nozzle diameters especially by the thermal ink jet printing process. In recent years, thermal ink jet printers have moved to higher jetting frequencies and smaller nozzle diameters to provide faster printing speeds with higher image quality. Thermal ink jet printers are now capable of printing (in drop volumes of 3 picoliters or less) at jetting frequencies in excess of 10 kHz and the need for higher frequency firing is a highly desirable feature. However, this high frequency firing often comes at the cost of variability in the firing velocity, which leads to poor image quality in the final printed image. In addition, the demands of current thermal ink jet printing require that the nozzles fire reliably for a large number of firings during the life-time of a printer. As an example, a typical ink jet nozzle may be required to fire in excess of 5×107, and up to as many as 1×109, individual firing events without malfunctioning or ceasing to fire altogether.
Another problem for drop-on-demand inkjet printing devices, especially those using pigment inks, is the recovery of a nozzle that has not been fired for a period of time such that the ink in the chamber has begun to dry out. This can occur during the time required to print a document if only certain inks are required for that document and the remaining inks remain idle. This phenomenon is typically referred to in the art as latency. Most ink jet printers will fire idle nozzles at specified intervals to maintain the reliable firing of all the jets. Unfortunately, pigment inks, and in particular pigment inks with high loads of pigment and polymers designed for high image quality and durability on the broadest range of media, can still show poor reliability even with reasonable idle-jet maintenance routines. If an ink requires excessive amounts of maintenance firing, this can also reduce the number of pages that can be printed from an ink tank, thereby reducing the efficiency of the tank and increasing the cost of printing.
Additional reliability problems can occur in situations where the printhead is left idle or uncapped for long periods of time and then is actuated again to eject ink. In some instances the idle printhead nozzles can partially clog or crust with ink components thereby degrading the ability of the printhead to eject properly. For example, the ink can be misdirected from the partially clogged nozzles or the drop velocity can be greatly diminished. In some instances, the nozzle will become permanently clogged and in other instances a lengthy and costly maintenance operation may be required to recover the nozzle back to a usable state of operation.
An important attribute for pigment-based inks, especially those with high loading of pigments and polymers is the ability of fresh ink to redissolve and redisperse ink that has dried in or around the nozzles. An ink that easily redissolves will readily recover even if the print head is allowed to dry, and will generally be more reliable during normal operation and latency conditions. The ability of an ink to redissolve is easily tested by allowing a specific amount of ink to dry out under specified conditions and then observe if that ink redissolves into a specified amount of fresh ink. An ideal ink will quickly redissolve all of the dried ink without forming visible particles or chunks. An ink with poor redissolution properties will not dissolve the dried ink or at best will only break it up into large chunks or particles that still may clog a print head nozzle.
Pigment-based inks formulated with polymeric dispersants and binders can be difficult to jet through inkjet printheads having small nozzle diameters especially by the thermal inkjet printing process. This is especially true of pigment-based inks, which are formulated with humectants or penetrants that lower dynamic surface tension.
Ink jet inks employing pigment dispersions that also include pyrrolidinone compounds to improve the jetting reliability are disclosed in Brust et al., US2009/0170986, Yatake, U.S. Pat. No. 7,981,947, and Saito et al., U.S. Pat. No. 8,013,035. The use of pigment levels in inkjet inks up to 10 wt % is generally described in the prior art, although use of pigment concentrations of 4 wt % and less are frequently employed to avoid printer reliability problems.